1. Bring large pot of water to boil.
2. Microwave butter in 13 x 9 x 2-inch microwave-safe dish on high 1 minute to melt. Stir in flour. Microwave on high 1 minute. Whisk in half-and-half, clam juice, salt, nutmeg and cayenne. Microwave on high 3 minutes. Whisk until smooth. Microwave, covered, on high power 4 minutes, until thickened.
3. Add pasta to boiling water; cook following package directions. Drain.
4. Meanwhile, add shrimp, scallops, clams, sherry and scallions to half-and-half mixture. Cover with microwave-safe plastic wrap, venting slightly. Microwave on high 3 minutes.
5. Uncover. Rotate dish; stir. Recover with plastic, venting. Microwave on high for 3 minutes.
6. Uncover. Rotate dish; stir. Recover, venting. Microwave on high 3 minutes. Let sit, covered, 5 minutes.
7. To serve, pour hot seafood mixture over cooked shells. Garnish with lemon slices, if desired. Makes 6 servings.